1. Field of the Invention
The present invention relates to a method of sintering compacts of powdered metals, ceramics, etc., to produce high-density sinters.
2. Description of the Prior Art
In the past, a hot isostatic press (hereinafter referred to as an HIP) process has been used for producing high-density sinters without any excessively large crystal grains from compacts of metal powders such as hard metal, high-speed steel, tool steel and stainless steel powders, powders of ceramics such as alumina, silicon nitride and silicon carbide and composite powers of hard metals, cermets, etc.
The application of this HIP process requires that a preliminary treatment is performed on the compact such that a high-temperature, high-pressure gas acts effectively in the densification of the compact. The known preliminary processing methods include the metal container process, ceramic mold process, glass bottle capsule process, glass powder capsule process, presintering process and the like and of these known processes the first four processes are not satisfactory from the standpoint of cost and productivity. Thus, the presintering process which presinters the compact so as to change the voids to filled pores in the compact to attain 94% of the theoretical density and then performs the HIP process is usually employed.
The usual presintering is disadvantageous in that a powder compact must be subjected for a long period of time to a temperature at which the growth of crystal grains progresses. The reason is that the sintering extends to the interior of the compact and therefore a high density of over 94% is required for completely filling the voids. There are another disadvantages that the mechanical properties of the sinter are deteriorated by the crystal grain growth and so on.